Process for prevent the formation of adhesives when annealing steel band

ABSTRACT

A process to prevent the formation of adhesives when annealing a steel band having a low carbon content uses an inert gas consisting of nitrogen and hydrogen and includes the phases of heating up, holding time and cooling. The process is characterized in that during the holding time, the steel band is coated by oxidation with a thin coating which is then completely removed by reduction during the cooling phase.

BACKGROUND OF INVENTION

The invention relates to a process to prevent the formation of adhesiveswhen annealing steel band having a low carbon content. Such process usesan inert gas consisting of nitrogen and hydrogen and has the phases ofheating up, holding time and cooling.

Steel band is annealed in the form of tight coils in pot furnaces,hood-type furnaces or continuous roller furnaces. An N₂ -H₂ gas mixtureor else an exothermic gas is normally employed as the inert gas.Adhesives are often formed when these steel bands are annealed.

These band adhesives are influenced by many factors. The main ones are:the geometry and dimensions of the surface roughness, the type of inertgas, the contact pressure, the temperature and the time.

The literature speculates that adhesives are formed at the site of thesteel surface where elevated pressure and a relative movement of thespirals occur during the cooling stage. As a result, adhesion anddiffusion phenomena take place.

SUMMARY OF INVENTION

The invention is based on the task of creating a process to prevent theformation of adhesives when annealing steel band having a low carboncontent.

In accordance with the invention, during the holding time, the steelband is coated by means of oxidation with a thin coating which is thencompletely removed by reduction during the cooling phase.

The steel bands are annealed (held) at temperatures ranging from 650° C.to 720° C. [1202° F. to 1328° F.].

The coating formed by means of the process according to the inventionserves to prevent the individual spirals from adhering to each other atthe beginning of the cooling phase, that is to say, up to a temperatureof 600° C. [1112° F.] in the core. Since this is when the tensionsinside the coil between the individual spirals are at their highestlevel, this limit temperature was designated as "critical".

When the temperature falls below this critical value, it is necessary toonce again create reducing conditions in the furnace in order tocompletely reduce the oxide coating thus formed as the annealingoperation proceeds.

This can be done by changing the water-gas equilibrium (hood underpressure) or by replacing the furnace atmosphere with, for example, N₂/H₂.

THE DRAWINGS

FIG. 1 is a graph showing the C-H-O three-substance system used todetermine the atomic composition of the inert gas at 680° C. [1256° F.];and

FIGS. 2a, b are graphs showing an example of the course of the annealingphases when the band steel is being annealed (680° C. [1256° F.]),dividing up the N₂ -H₂ -CO₂ inert gas into two theoretical gas mixtures.

DETAILED DESCRIPTION

In FIG. 1, Point 1 describes an N₂ -H₂ gas mixture, Point 2 anexothermic gas and Point 3 an N₂ -H₂ gas mixture with the addition ofCO₂.

FIG. 2a refers to the theoretical H₂ -H₂ O gas mixture, which isresponsible for the reduction. FIG. 2b refers to the theoretical CO-CO₂gas mixture, which is responsible for the oxidation (holding time untilapproximately 600° C. [1112° F.])/reduction (T<600° C. [1112° F.]).

In the case of gas mixtures consisting of carbon monoxide (CO), carbondioxide (CO₂), hydrogen (H₂) or else methane (CH₄), there is a reactionamong the components until a uniform carbon activity is reached.

If there is no equilibrium between the metal surface and the gas phase,carbonization or decarbonization reactions or else oxidation orreduction reactions bring about a mass transfer between the two phasesuntil a state of equilibrium is reached.

Thus, every instance of carbon activity resulting from the desiredchemical composition of the steel surface--in a state of equilibrium ata defined temperature--is associated with a certain gas composition.

    [C]+-[CO.sub.2 ]-→2-[CO]-

    [C]+-[H.sub.2 O]-→-[CO]-+-[H.sub.2 ]-

Since the carbon activity for low-alloy steel has to be low and in thiscase, the oxidation/reduction reactions are important, the gascomposition was associated with the homogenous water-gas reaction, whichis a compilation of the following reactions: ##STR1##

At a defined temperature, the corresponding state of equilibrium isemployed to reach a certain gas composition.

For example, an N₂ -H₂ (97%:30%) gas mixture was selected. With theaddition of a certain quantity of CO₂ and the total quantity of inertgas, the course of the water-gas reaction at certain temperatures iscontrolled. In the homogeneous state, the following reactions take placeunder the hood:

    H.sub.2 O=H.sub.2 +1/2O

    CO.sub.2 =CO+1/2O.sub.2

    CO.sub.2 +H.sub.2 =H.sub.2 O+CO

The transfer of oxygen then causes the following:

    Me+1/2O.sub.2 =MeO

    Me+CO.sub.2 =MeO+CO

Since the reaction CO₂ →CO+1/2O₂ is relatively slower in comparison toH₂ O=H₂ +1/2O, longer times for the oxidation in the CO-CO₂ gas mixturehave to be expected.

EXAMPLE

The following generally applies to the homogenous water-gas reaction:

    Lg K.sub.w =Lg (P.sub.CO ·P.sub.H.sbsb.2.sub.O /P.sub.CO.sbsb.2 ·P.sub.H.sbsb.0)=1717/T+1.575

At, for instance, 680° C. [1256° F.], K_(w) =0.6.

If, for example, a gas mixture consisting of 1.2% CO₂ and 3.0% H₂ and0.004% H₂ O is heated up to 680° C. [1256° F.], the question arises asto the gas composition with which the equilibrium was established. Thereaction equation in a homogenous system follows the relationship:

    V.sub.A A+V.sub.R B+ . . . +ΔH=V.sub.B E+V.sub.F F+ . . .

wherein Vi, i=-[A, . . . F]- stands for stoichiometric molar numbers ofthe substances i.

When using the molar fraction Xi=Pi/P, the law of mass action acquiresthe following form:

    E.sup.VE ·F.sup.VF /A.sup.VA ·B.sup.VB =K.sub.p ·P.sup.exp-ΔΣ.spsp.Vi

    E.sup.VE ·F.sup.VF /A.sup.VA ·B.sup.VB =K.sub.p ·.sub.p exp-ΔΣVi

The reaction index ΔΣVi, i.e. the sum of the molar numbers of theinitial products minus the sum of the molar numbers of the finalproducts, is as follows:

    ΔΣVi=V.sub.B +V.sub.P -V.sub.A -V.sub.B

and it provides information on the volume change and pressuredependence.

In the above-mentioned example of the water-gas reaction, the result isthat ΔΣVi=0, whereby the following generally applies:

    K.sub.P P·P.sub.exp -ΔΣVi=K.sub.C (RT/P) exp ΔΣVi

Since ΔΣVi=0, it follows that K_(P) =K_(C). Consequently, the reactionis not dependent on the pressure.

If the following is established for the original gas composition:

X_(CO) =0;

X_(H).sbsb.2 =0.03;

X_(CO).sbsb.2 =0.012;

X_(H).sbsb.2_(O) =0.00004

and the molar fraction of the newly formed CO: =Z, the following resultsfor the equilibrium composition of the molar fraction:

    CO:=Z

    CO.sub.2 :=X.sub.CO.sbsb.2 -Z

    H.sub.2 :=X.sub.H.sbsb.2 -Z

    H.sub.2 O:=X.sub.H.sbsb.2.sub.O +Z

Then the mass action law is as follows:

    K=Z(X.sub.H.sbsb.2.sub.O +Z)/(X.sub.CO.sbsb.2 -Z)(X.sub.H.sbsb.2 -Z)

Solved according to Z, the following polynomial results:

    (1-K)Z.sup.? +(X.sub.H.sbsb.2.sub.O +KX.sub.CO.sbsb.2 +KX.sub.H.sbsb.2)Z-KX.sub.CO.sbsb.2 X.sub.H.sbsb.2 =0

if the value 0.6 is selected for K (680° C. [1256° F.]), the resultinganalysis of the ideal state is as follows;

H₂ =2.24%,

CO=0.76%,

CO₂ =0.44%

H₂ O=0.77%

With K=0.01, for example, the following would apply:

H₂ =2.83%,

CO=0.17%,

CO₂ =1.03%

H₂ O=0.17%

Theoretically, the composition of the gas mixture can vary within thefollowing ranges:

H₂ =2.24% to 2.83%

CO=0.17% to 0.76%

CO₂ =0.44% to 1.03%

H₂ O=0.17% to 0.77%

In this case, the following was measured during an experiment:

H₂ =2.1%,

CO=0.78%,

CO₂ =0.86%,

H₂ O=0.06%

This gas composition corresponds to a certain Point 3 in the C-H-Othree-substance system (FIG. 1).

The position of the point in the three-substance system determines theinfluence of the gas composition on the surface of the band steel.

Consequently, the addition of CO₂ to an N₂ -H₂ gas mixture shifts thecorresponding Point 1 of the gas composition in the three-substancesystem from a reduction range to a border range of oxidation. Dependingon the method of operation of the furnace, the gas composition changesso that the water-gas equilibrium can vary between 0.01 and 0.6.

In this sense, an optimum concentration of the CO₂ utilization was foundin order to fully utilize properties of the water-gas reaction forpurposes of achieving annealing free of adhesives.

This is achieved using CO₂, for instance, 0.9% to 2.5% in the 97:3 N₂-H₂ gas mixture, that is, with relatively low CO₂ contents in comparisonto exothermic gas.

Additional theoretical considerations have shown that an oxide coatingor a CO₂ accumulated coating can be formed as a protective coating inthe molecular range of the surface of the steel band, and this coatingprevents adhesion of the spirals. In order to achieve this, a slightlyoxidizing atmosphere has to be formed in the furnace during or at theend of the holding time during annealing, and this atmosphere creates athin inactive coating (FeO) on the surface of the band steel.

FIG. 2 represents the N₂ -H₂ -CO₂ inert gas atmosphere in all annealingphases. Here, these were theoretically divided up into:

a) H₂ -H₂ O-gas mixture

b) CO-CO₂ -gas mixture

Through the homogeneous water-gas reaction or through the two partialreactions, as described above, CO and H₂ O are formed in such quantitiesthat the CO-CO₂ gas mixture is responsible for the oxidation above 600°C. [1112° F.].

The H₂ -H₂ O gas mixture, on the other hand, has a reducing effect. Asthe temperature drops (cooling phase), the CO-CO₂ relationship of theinert gas which is present changes in such a way that a completereducing force of the two gas mixtures is only utilized at a temperaturebelow 600° C. [1112° F.].

The oxide coating formed is reduced at the end of the cooling phase.

The optimum use of CO₂ involved the entire surface of the annealedproduct and it amounts to 0.2 to 0.3 grams of CO₂ per m² of steel bandsurface.

The process according to the invention makes it possible to prevent orelse to drastically reduce the formation of adhesives and to replace thegeneration of exothermic gas with synthetic gases. In comparison toexothermic gas with approximately 8% CO and 6% CO₂, it can be said to bean environmentally safe process since the emission of CO is reduced byapproximately 95%, while the emission of CO₂ is reduced by about 92%.

SUMMARY

It is often the case the adhesives are formed on the surface when steelband having a low carbon content is annealed. In order to prevent this,the steel band is coated by means of oxidation with a thin coating at atemperature above 600° C. [1112° F.] (holding time), and this coating isthen removed by reduction at a temperature below 600° C. [1112° F.]during the cooling phase. In the case of an inert gas consisting ofnitrogen and hydrogen, carbon dioxide is preferred as the oxidationmedium. The reduction takes place by changing the water-gas equilibrium.

I claim:
 1. Process to prevent adhesion during the annealing of steelbillets having a low content of carbon in an inert gas atmosphereconsisting of 95% to 99% nitrogen and the rest of hydrogen; the processincluding sequentially heating up, maintaining and cooling off phases,the improvement being in that during the maintaining phase the steelbillet is coated in a coating step with a thin covering layer by meansof oxidation at temperatures above 600° C. [1112° F.], during thecoating step 0.2 to 0.3 grams of CO₂ are added to the inert gas persquare meter of the surface of annealing material, and completelyremoving the covering layer through reduction during the cooling offphase at temperatures below 600° C. [1112° F.].
 2. Process according toclaim 1, characterized in that the reduction takes place by a change inwater equilibrium during the cooling off phase.